1. Field of the Invention
This invention relates to emergency protection circuits operating in conjunction with a normally stable power supply and more particularly, protection circuits of normally stable power supplies that operate in conjunction with a system of high intensity, gaseous-discharge lamps and therfore susceptible to injury from a supply voltage exceeding even a momentary voltage level beyond a certain limit.
2. Description of the Prior Art
There are many fuses, circuit breakers and the like that operate in the presence of a voltage overload to open an incoming supply source from injuring a delicate electrical device and/or electrical circuit. These protection devices are satisfactory when protecting devices and circuit components which themselves are capable of tolerating, at least for a short period of time, a circuit overload condition without appreciable injury, such as might be caused by the application of high voltage and attendent high current caused by a short circuit.
There are, however, circuits and devices that are incapable of tolerating even a momentary overload condition and, therefore, such protection devices and/or circuits are not satisfactory in preventing injury from occurring. There are many other installations where, although the protected devices or components can tolerate a bit of overvoltage, their life or subsequent performance is impaired to some extent when subjected to a momentary surge. That is, a surge may not cause their failure, but will degrade their vitality.
One situation that can cause an unexpected surge to occur is one involving electrical storage devices, such as capacitors and inductors, that can discharge or untimely cause a current or voltage to occur back into a power supply network, thereby causing related devices connected to the power supply to experience overvoltage or overcurrent conditions even through the source or supply voltage remains within limits. A circuit which is suitable for providing some protection under such conditions is disclosed in FIG. 7 and related illustrations of copending patent application Ser. No. 654,926, filed Feb. 2, 1976, by the same inventor. A phototransistor controlled by the brightness of a light emitting diode connected across the dc output of the power supply determines the application of gate voltage to a triac. This conduction timing "regulates" the output even though there is no regulation of the ac input to the power supply. The circuit is protected against surges of ac input by the gating of another triac that operates a coil connected for opening its contacts in series with the source line.
Although the circuit does open the line in the presence of sustained overvoltage input conditions, its operation is somewhat hesitant when the voltage conditions are borderline. Further, the dc timing connection does provide regulation in the case of a dc voltage output above a predetermined danger level, but the action does not ensure that a sudden surge will result in opening the power circuit. That is, there is no absolute protection provided.
Therefore, it is a feature of the present invention to provide an improved electronic protection circuit for monitoring either or both ac and dc voltage conditions and providing a positive output for disconnecting a power circuit in the presence of a detected overload, even a borderline detected overload.
It is another feature of the present invention to provide an improved electronic protection circuit operating in the above manner in connection with photodiodes controlling the conduction of phototransistors, which, upon conduction, discharge previously stored charges for producing a large overvoltage signal to the power supply disconnect network.